Usually, in order to increase the reactivity of the raw meal, the sieve residues are reduced to the 80 .mu.m and 200 .mu.m sieve, but this leads to an increase in the energy consumption and the operating costs for grinding the raw meal.
It is known that, below a reactivity of the raw meal which is still kept relatively high, the sieve residues on the 80 .mu.m sieve can be raised from approximately 10% up to 20% and on the 200 .mu.m sieve from approximately 1% up to 5%. This corresponds to a grain size ratio of the 0.01-80 .mu.m grain size to the 80.01-500 .mu.m grain size of 9:1 to 4:1. Nevertheless, this is associated with high energy consumption and high operating costs for grinding the raw meal.
The object, therefore, is to set the grain class ratio of the raw meal, particularly of the basic raw meal component, that is to say to grind up the raw meal extremely coarsely, so that the reactivity of the raw meal under extreme coarse grinding is not hindered but favoured, and so that under the extremely changed grain class ratio of the raw meal a method of producing cement by grinding, drying, mixing and then roasting the raw meal is provided with which the necessary energy consumption, and likewise the operating and investment costs for the least possible grinding assemblies required from the point of view of apparatus, are reduced, particularly in the case of grinding raw meal, with a simultaneous increase in the throughput of the raw meal apparatus and the kilns as well as simultaneous reduction of the specific fuel requirement.
The object is achieved by a first example with a method of producing cement in which the basic raw meal component is ground to a grain class ratio of the 0.01-80 .mu.m grain class to the 80.01-500 .mu.m grain class of 1.5:1 to 1:9 so that before the formation of the clinker melt in the sintering zone apart from the conventional topochemically formed C.sub.2 AS, C.sub.3 A, C.sub.12 A.sub.7 and C.sub.4 FA, the easy to melt CS and C.sub.3 S.sub.2 are largely produced instead of the difficult to melt C.sub.2 S.
By this measure it is possible to raise the clinker melt content in the sintering stage and to lower the melting temperature, which accelerates the clinker formation and lowers the sintering temperature during the clinker formation. Naturally, in this case the energy consumption during grinding of raw meal is considerably reduced and the output of the grinding plant is increased. The cement properties are improved by the better mineral formation particularly of alite and belite. The discharge of dust from the kiln is reduced thereby, so that the heat loss from the kiln is reduced.
The phenomena which occur in this case are explained as follows:
The clinker formation may be considered as a function of the state of matter (liquid or solid) in the two stages of development. The first stage of development of the mineral formation consists of the topochemical reactions (solid state reactions) and lasts up to 1250.degree. C. The second stage of development of the clinker formation begins at 1250.degree. C. and is principally completed above a temperature of 1300.degree. C. because of the melting process.
It is known from the prior art that in order to lower the fuel requirement during clinker burning and to raise the quality of the clinker it is advantageous to accelerate the rate of mineral formation not only through the melt but also through topochemical reactions. Usually the increased reactivity of the raw meal with unchanged LS II is achieved by relatively fine grinding, relatively low SM and TM and by the use of mineralisers.
The existing conceptions of an optimum fractional composition of the raw meal, i.e. that the content of the particles greater than 80 .mu.m should not amount to more than 10-15%, are not fully substantiated.
As evidence it may be mentioned that due to the solid state reactions between the calcite and SiO.sub.2 carrier particles smaller than 60-80 .mu.m premature belite formation because of an increasing time and temperature interval between the alite formation and the belite formation leads to the crystal growth of the topochemically formed belite and of the residual free lime. As a result the dissolving of the recrystallised belite and of the recrystallised free lime in the melt is hindered, which causes a delay in the alite formation, i.e. requires an increase in the sintering temperature. It follows, therefore, that an acceleration of the topochemical reactions in general does not always lead to an increase in the rate of clinker formation. The clinker formation may even be hindered.
The question arises as to how the clinker formation may be optimised.
It is known that the rate of mineral formation through the melt is approximately 10,000-100,000 times higher than through the topochemical reactions. From this it may be concluded that a delay in the topochemical reactions can frequently be made up for at the stage of development of the reactions in the melt. This means that latent reserves for increasing the throughput of the kiln are also located in the sintering zone. It is obvious that at an increased throughput of the kiln, with the other conditions remaining the same, because of a reduction of the specific heat losses the lowest specific energy requirement for the clinker formation can be achieved. For this reason numerous attempts have been made to accelerate the clinker formation and to increase the proportion of clinker melt. However, with the conventional technology for cement production the possibilities described above are greatly restricted because of the necessary properties of the cement and the resulting necessary chemical compositions of the raw meal.
It is, moreover, very important that the rate of clinker formation through the melt is in practice not very dependent upon the size of the particles, particularly of calcite. This is confirmed elsewhere, for example in the crushed stone technology for producing the cement clinker and in metallurgical processes in which the reactions proceed completely in the melt, although the grain size fractions of the starting raw materials may be very coarse-grained (up to 50 mm).
Because of this it may be concluded, contrary to the known conceptions, that hindering of the clinker formation through the melt is not dependent upon the size of the calcite particles (limestone particles) but upon the diversion of the Ca.sup.2+ ions which are located at the boundary between the melt and solid phases, i.e. at the boundary layer.
Until the boundary layer is saturated as regards the Ca.sup.2+ ions the dissolution of the solid free lime phase in the melt is hindered.
The elimination of the negative retarding effect of the boundary layer which is saturated with Ca.sup.2+ ions on the rate of clinker formation can only be achieved by an increase in the reaction surface between the solid and melt phases. For this it is necessary to increase the proportion of melt.
With an increased proportion of clinker melt a mechanical stress is very helpful, since in this case by comparison with the proportion of clinker melt which is reduced below the usual eutectic an effective abrasion of the saturated zone takes place. Thus the slowest reaction stage is accelerated through the melt. As a result the clinker formation is considerably accelerated, particularly in the case of raw meal containing the coarsely ground limestone.
In the question which results from this, "How can the proportion of melt be increased?", it helps us to consider the phase diagram in the C-S-A and C-S-A-F systems.
The analysis of the 3 and 4 substance system shows that for the raw meal, the principle components of which are CaO, SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3, apart from the usual eutectics present at the C-A margin for the production of both grey and white Portland cement (C.sub.3 S--C.sub.2 S--C.sub.3 A--C.sub.4 AF or C.sub.2 S--C.sub.3 A--C.sub.12 A.sub.7 --C.sub.4 AF; grey) (C.sub.2 S--C.sub.3 A--C.sub.12 A.sub.7 ; white) there are eutectics still lying opposite on the C-S margin with the same or a lower melting point (CS--CAS.sub.2 --S; C.sub.3 S.sub.2 --CS--C.sub.2 AS; C.sub.3 S.sub.2 --CS--C.sub.2 AS--C.sub.4 AF). As is shown from the existence ranges of the eutectics to be newly considered for sintering of the clinker, they are only formed in the presence of CS and C.sub.3 S.sub.2 instead of the C.sub.2 S.
From the comparison of the chemical compositions of the available eutectics it follows that the formation of the eutectics not to be utilised for the clinker sintering is possible with falling or the same melting points due to a considerable increase in the SiO.sub.2 fraction with the same or a reduced CaO fraction in the mixture. It may be recognised that the ratio between acidic oxides in the eutectics to be newly considered in precise contrast to the usual eutectics corresponds almost completely to the raw meal for Portland cement production. There is every reason to assume that in the coarse of the linker formation due to the aforementioned eutectics which are to be newly considered all acidic oxides of the raw meal without exception are included completely with a corresponding CaO fraction in the melt. With regard to the conventional technology, in which the clinker melt composition corresponds to the highest fraction of Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3, it has also already been demonstrated that whilst the Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 oxides of the raw meal go over almost completely into the melt, the SiO.sub.2 oxide can only be partially (less than 10% of the total content of SiO.sub.2) dissolved in the melt. However, on the basis of the new theoretical conceptions this is only possible when the predominant part of the CaO is capable of solid state reactions.
Because of this it may be expected that the proportion of melt in the course of the clinker formation can be considerably increased by the new eutectics. This results from the following explanations and calculations, in which the reaction sequences are explained on the one hand with the eutectics usually used as a basis and on the other hand by way of the new eutectics.
As already follows unequivocally from the value of the SM, the SiO.sub.2 fraction in the raw meal is much greater than the Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 in total. Because the SiO.sub.2 oxide of the raw meal according to the new technology is chemically bound to the easy to melt silicates, such as CS, C.sub.3 S.sub.2 and C.sub.2 AS, in order to form the new eutectic, the proportion of melt in the sintering zone can be considerably increased with simultaneous formation of usual melt-forming minerals (C.sub.3 A, C.sub.4 AF).
The proportion of melt which could theoretically be produced in the clinker formation in the sintering zone based on the new and the usual eutectics of the phase diagram has been calculated for two grey clinkers which differ in their SM and TM as well as for a white clinker.
The calculated data show that the newly considered eutectics (CS--CAS.sub.2 --S; C.sub.3 S.sub.2 --CS--C.sub.2 AS; C.sub.3 S.sub.2 --CS--C.sub.2 AS--C.sub.4 AF) are much more favourable with regard to the proportion of clinker melt than the usual eutectics of the grey and the white clinker (C.sub.3 S--C.sub.2 S--C.sub.3 A--C.sub.4 AF or C.sub.2 S--C.sub.3 A--C.sub.12 A.sub.7 --C.sub.4 AF; grey) (C.sub.2 S--C.sub.3 A--C.sub.12 A.sub.7 ; white). This means that in the alite formation due to the new eutectic the proportion of melt in the sintering zone can be temporarily increased for the white cement clinker from approximately 11% to 21-23% and for the grey cement clinker from approximately 21% to 45-47%.
This again effects a considerable acceleration of the crystal-chemical conversions. This means that it is most favourable if the chemical composition of the melt for producing the white cement clinker corresponds the eutectic in the system C.sub.3 S.sub.2 --CS--C.sub.2 AS and the composition for the grey clinker corresponds to the eutectic in the system C.sub.3 S.sub.2 --CS--C.sub.2 AS--C.sub.4 AF.
From this the question follows: If a quite different eutectic is more favourable than that of the conventional technology, why is the progress of the alite formation itself set by way of the usual eutectic?
In order with regard to this question to point to the appearance of the clinker formation, we consider below the mechanism and the kinetics of the clinker formation.
The present experimental and thermodynamic studies show that the mineral which forms first in the heterogeneous CaO--SiO.sub.2 system independently of the CaO:SiO.sub.2 ration under metastable conditions is C.sub.2 S. Accordingly, with a CaO:SiO.sub.2 molecular ration of 1:1 or 3:2, CS or C.sub.2 S.sub.2 can only be formed after the proportion of the CaO which is capable of solid state reaction, the Ca.sup.2+ ions of which are the most mobile, is completely fixed. This means that the desired CS or C.sub.2 S.sub.2 formation in the CaO--SiO.sub.2 system is the slowest, i.e. requires a specific time. Because of this the immediate formation of the newly considered eutectics (which corresponds to the CS--CAS.sub.2 --S or C.sub.3 S.sub.2 --CS--C.sub.2 AS or C.sub.3 S.sub.2 --CS--C.sub.2 AS--C.sub.4 AF system) in the raw meal for Portland cement production cannot be achieved, since in the heating under metastable conditions it is heated to the necessary temperature. In this case the main cause is the lack of the easy to melt CS, C.sub.3 S.sub.2 and C.sub.2 AS silicates which are necessary for the newly considered eutectic instead of the C.sub.2 S. This relates likewise to the heterogeneous CaO--SiO.sub.2 mixture with a CaO:SiO molecular ratio of 1:1 and to a heterogeneous mixture of one of the newly considered eutectics prepared from the CaO, SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3 oxides. This means that the melting of a heterogeneous mixture, the chemical composition of which corresponds as regards the principal components (CaO, SiO.sub.2, Al.sub.2 O.sub.3 and Fe.sub.2 O.sub.3) to one of the newly considered eutectics, at a corresponding temperature could only take place after the complete chemical fixing of all components. So long as the melting point of the eutectic located in the CS--CAS.sub.2 --S system lies in the temperature range of the simultaneous formation of C.sub.2 S, C.sub.3 S.sub.2 and CS, and the C.sub.2 S forms first amongst the silicates, the formation of this eutectic which occurs at 1170.degree. C. is hindered until the C.sub.2 S is completely converted to CS. In this case the available CaO fraction capable of the solid state reaction must correspond to the eutectic in the CS--CAS.sub.2 --S system. In practice this means that during the heating of the raw meal in the kiln under metastable conditions the formation of the eutectic in the CS--CAS.sub.2 --S system is not possible.
With regard to the stability and to the necessary time which the C.sub.2 S.fwdarw.CS conversion could ensure during the heating of the raw meal in the kiln, the eutectics in the C.sub.3 S.sub.2 --C.sub.2 AS--CS and C.sub.3 S.sub.2 --C.sub.2 AS--CS--C.sub.4 AF systems with the melting points of 1310.degree. C. and approximately 1260.degree. C. are therefore to be regarded as the most favourable for clinker formation.
Under the explained conditions the formation of the chosen eutectic in the C.sub.3 S.sub.2 --C.sub.2 AS--CS and C.sub.3 S.sub.2 --C.sub.2 AS--CS--C.sub.4 AF system by topochemical reactions in the raw meal for the production of cement is only possible when the proportion of finely ground calcite particles, i.e. particles which are capable of solid state reactions, is sufficient, apart from the C.sub.12 A.sub.7 (C.sub.3 A) and C.sub.4 AF, only for the formation of CS and C.sub.3 S.sub.2 instead of the C.sub.2 S as well as for C.sub.2 AS, and when an optimally sufficient time is available for the C.sub.2 S.fwdarw.CS conversions.
So long as the solid state reactions can further be ensured by the presence of the calcite particles and the particles of acidic minerals smaller than 80 .mu.m, in the production of raw meal the content of calcite particles smaller than 80 .mu.m should only be available for the formation of C.sub.3 S.sub.2, CS, C.sub.2 AS as well as C.sub.12 A.sub.7 (C.sub.3 A) and C.sub.4 AF. In order to be incapable of solid state reactions, after calculations have been carried out the rest of the calcite-containing components of the raw meal must be present in a quantity of approximately 40-80% in the particles over 80 .mu.m.
Only in this case is the clinker melt formed from CS, C.sub.3 S.sub.2 and C.sub.2 AS in addition to the C.sub.3 A and C.sub.4 AF.
This in turn means that the proportion of the melt is increased, so that the rate of clinker formation is accelerated. The coarse-grained calcite which is deacidified later in time enters the sintering zone in the state of finely crystalline highly reactive free lime. This free lime sinters and granulates with the clinker melt better than the recrystallised free lime with coarsely crystalline structure which in the known technology was produced from fine-grained calcite particles by crystal growth of the free lime formed earlier.
As a consequence of this a reasonable ratio must be set between the particles which are capable of solid state reaction and the particles which are capable of melting reaction.
It is obvious that the particles of the substances which contain clay, silicic acid, quartz and iron, which are regarded as acidic components, must also be capable of solid state reaction. In order to be capable of solid state reaction, the particles of the acidic raw meal component must also be correspondingly finely ground. Usually the particles of the acidic raw meal components must be below 80 .mu.m and even lower.
The optimisation of the fractional composition of the raw meal must, as already explained above, be targeted so that the proportion of the particles of calcite which are capable of solid state reaction were only sufficient for fixing the acidic raw meal components to C.sub.3 A, C.sub.4 AF, C.sub.2 AS as well as CS or C.sub.3 S.sub.2.
The coarsening of the raw meal changes not only the progress of the solid state reaction but also the mechanism for formation of the alite from the melt. Alite is not formed via the reaction in the melt of the sintering zone between the topochemically formed belite and the residual free lime, but it crystallises as does the belite directly out of the melt, with the residual lime included, without formation of the topochemical belite immediately before the melt formation. As as result a lower energy requirement is necessary for formation of the alite.
Expressed more simply, this means that the formation of belite in the first stage of the clinker formation (the solid state reactions) is largely avoided by the corresponding reduction in the fineness of limestone components.
In this case it is particularly desirable that the clinker formation in the case of an unchanged alite content (LS II) preferably takes place in the melt, without increasing the Fe and Al oxide fraction or using mineralisers.
With the aid of the method according to the invention it has been shown that with an optimisation of the fractional composition of raw meal a targeted influence on the mechanisms of the clinker formation can be achieved, by which the proportion of clinker melt can be increased. The grinding of the raw meal should be linked to the clinker burning process so that the remaining portion of the raw meal to be converted in the melt is more coarsely ground.
Several embodiments of the invention are set out or described in greater detail below.